Some research is needed to bring that price down. A plumber has to find someone to dig the ditch. Sometimes you can find someone that works for less or even the rentle companies will supply an operator. On our new construction it is labourers that installl the pipe from the road to the house but it is inspected by the city. Often the best bet is to0 find the people that work on new houses, find some builders and get some phone numbers. Make sure who ever digs calls first, you want to know where water and gas pipes are.

First bid came in at $8700. I suggested I handle excavation and he insisted that its a 9' deep trench that he needs to handle shoring for safety reasons, thus wont leave that to me, or let me sub that work out personally.

Moving on to the next company; have two more contractors coming to look at it this week to make bids.

Yeah; I just want to be sure I'm not being taken for a ride. Any job over a thousand or two I tend to get at least 3-5 bids. We had a fence done and I had quotes for the same fence range from $4800 up to $9000; I'm expecting some variance in quotes again here (obviously)

When you said you had a dip in the line, how bad did it look on the scope? What’s the overall condition of the drain pipe going out to the street? Can just that spot be fixed?

You said the plumber advised that it most likely would be ok if you were careful. I just wonder with a good cleaning full length you might be ok for a long time. I would risk it if it were mine if I didn’t have plans for the basement beyond some storage based on the price to redo it. I have items in the house we just bought that are on the watch list. The furnace was on the replace list but it worked great all winter and when I had it inspected I got kind of a mixed reply like you did on replacing it.

It's in okay shape. It's old corroded cast iron, but no breakages/etc. I think if we're excavating (with the full length being 18' and the problematic section being 9'), its probably worth doing it all for future prevention.

Right now we're actively avoiding feminine hygiene products, and watching it. If it keeps happening we're going to pull the trigger, but if that alleviates the issue in the interim, this will go on the back burner for a bit. Unfortunately there's no quick way to find out (short of taking a few big bowel movements and hoping I dont have to shopvac the basement again..)

edit: I'm just going to setup a burn barrel in the back yard and insist we no longer use the bathrooms in the house

Yes I was picturing a 100 foot of line dropping down to that depth. If it’s that short of a distance I would do it all also. That’s $480 per foot sounds kind of high. What I have found is a clog will start where something causes a snag and gets caught. The rotor rooter type machines have a lot of different tools they can run thru and smooth it up some. Might be worth a shot.

Interestingly; I've called probably 10+ plumbers now to get quotes. Of those ten, one looked up property lines in the city system and suggested that based on my description, that the responsibility lies with the city.

He said he'd come out for $450 to camera and mark it plus handle calling the city/coordinating replacement.

I'm skeptical because he's the only one who has suggested this is even possible, so right now I'm working on getting in touch with the local water authority to verify his claims. If he's actually right, then $450 is a hell of a bargain compared to doing this on my own dime.

Go to the city planning dept and look at the site map for your area, and the sewer plan for your area, that's all that guy is going to do for 450.
Remember the city workers work for you, put them to work.